U.S. patent number 7,568,100 [Application Number 08/999,766] was granted by the patent office on 2009-07-28 for steganographic method and device.
This patent grant is currently assigned to Wistaria Trading, Inc.. Invention is credited to Marc Cooperman, Scott A. Moskowitz.
United States Patent |
7,568,100 |
Moskowitz , et al. |
July 28, 2009 |
**Please see images for:
( Certificate of Correction ) ** |
Steganographic method and device
Abstract
An apparatus and method for encoding and decoding additional
information into a stream of digitized samples in an integral
manner. The information is encoded using special keys. The
information is contained in the samples, not prepended or appended
to the sample stream. The method makes it extremely difficult to
find the information in the samples if the proper keys are not
possessed by the decoder. The method does not cause a significant
degradation to the sample stream. The method is used to establish
ownership of copyrighted digital multimedia content and provide a
disincentive to piracy of such material.
Inventors: |
Moskowitz; Scott A. (Tokyo,
JP), Cooperman; Marc (Palo Alto, CA) |
Assignee: |
Wistaria Trading, Inc. (Sunny
Isles Beach, FL)
|
Family
ID: |
23942708 |
Appl.
No.: |
08/999,766 |
Filed: |
July 23, 1997 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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08775216 |
Dec 31, 1996 |
5687236 |
|
|
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08489172 |
Mar 18, 1997 |
5613004 |
|
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Current U.S.
Class: |
713/176; 726/32;
705/57; 380/46; 380/208; 380/207; 380/205 |
Current CPC
Class: |
G06T
1/0021 (20130101); H04N 21/835 (20130101); H04N
21/2541 (20130101); H04N 1/32187 (20130101); H04N
21/4348 (20130101); H04L 9/3247 (20130101); G10L
19/018 (20130101); H04L 9/0656 (20130101); H04N
21/8358 (20130101); H04L 9/0643 (20130101); H04N
21/8352 (20130101); H04L 9/12 (20130101); H04N
21/23892 (20130101); H04L 9/3263 (20130101); H04N
7/1675 (20130101); H04N 1/32245 (20130101); H04N
1/32149 (20130101); H04N 1/3216 (20130101); H04N
21/23614 (20130101); H04L 2209/608 (20130101); G06T
2201/0064 (20130101); H04N 2201/327 (20130101); H04N
2201/3205 (20130101); H04L 2209/08 (20130101); H04L
2209/605 (20130101); H04N 2201/3233 (20130101); H04N
2201/3281 (20130101); G11B 20/00891 (20130101); H04L
2209/04 (20130101); H04N 2201/3226 (20130101) |
Current International
Class: |
H04L
9/00 (20060101) |
Field of
Search: |
;380/53,54,205,207,208,46 ;713/176 ;705/57 ;726/32 |
References Cited
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0372601 |
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EP |
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0565947 |
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EP |
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0581317 |
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EP |
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000581317 |
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0649261 |
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EP |
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EP |
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NL |
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WO 99/62044 |
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Dec 1999 |
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WO |
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WO 9963443 |
|
Dec 1999 |
|
WO |
|
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|
Primary Examiner: Smithers; Matthew
Assistant Examiner: Callahan; Paul
Parent Case Text
This is a division of application Ser. No. 08/775,216 filed 31 Dec.
1996 now U.S. Pat. No. 5,687,236, which is a division of
application Ser. No. 08/489,172 filed 7 Jun. 1995, now U.S. Pat.
No. 5,613,004 issued 18 Mar. 1997.
Claims
What is claimed is:
1. A method for steganographically protecting a digital signal
comprising the step of: a) providing a carrier signal that has been
encoded with independent information; b) using a stega-cipher to
steganographically decode independent information including a
digital watermark from the carrier signal; c) generating a first
derivative encoded signal representing the original carrier signal
combined with the encoded independent information, including the
digital watermark, wherein the first derivative encoded signal is
an arbitrarily close approximation of the original carrier signal;
and d) modifying the first derivative encoded signal to produce a
second derivative encoded signal, wherein the second derivative
encoded signal differs from the original carrier signal by a
greater degree than the first derivative encoded signal differs
from the original carrier signal, as measured by an arbitrary
signal metric.
2. A method for steganographically protecting a digital signal
comprising the step of: a) providing a carrier signal; b) using a
stega-cipher to steganographically encode independent information
including a digital watermark into the carrier signal; c) selecting
a mask set, said mask set including one or more random or
pseudo-random series of bits, referred to as masks; d) selecting a
random or pseudo-random start of message delimiter; and e)
selecting independent information to be encoded.
3. The method according to claim 2, wherein the carrier signal
includes a stream of digital samples.
4. The method according to claim 2, wherein the carrier signal
includes a continuous analog waveform.
5. The method according to claim 2, wherein the digital watermark,
includes at least one selected from the group consisting of: rights
ownership identification, authorship identification of the encoded
carrier signal, ownership identification of a unique copy of the
encoded carrier signal, and a serialization code uniquely
identifying a copy of the encoded carrier signal.
6. The method according to claim 2, further comprising the step of:
using a map table to define where watermark information is to be
encoded into the carrier signal based on the random or
pseudo-random masks, wherein the map table is defined such that any
index of the map table enables encoding of information.
7. The method according to claim 2, further comprising the step of:
f) generating a message bit stream to be encoded such that the
stream includes: 1) the random or pseudo-random start of message
delimiter; 2) a number of message bytes to follow the message; and
3) the independent information.
8. The method according to claim 7, wherein the independent
information contains, at least one selected from the group
consisting of: a hash value computed on the start of message
delimiter, and a digital signature of the start of message
delimiter.
9. The method according to claim 7, further comprising the step of:
g) separating an input sample stream into smaller discrete sample
windows comprising segments of the input sample stream.
10. The method according to claim 9, further comprising of the step
of: h) using positions within the sample windows and a position
within the input stream to index random or pseudo-random masks and
compute a mapping function to determine encoding positions and
encode digital watermark information into the sample windows.
11. The method according to claim 10, further comprising the step
of: i) computing a spectral transform of the sample windows prior
to digital watermark data encoding.
12. The method according to claim 11, further comprising the step
of: j) computing an inverse spectral transform of the encoded
spectral transform data after digital watermark data encoding.
13. The method according to claim 2, further comprising the step
of: adding unique data to each individual watermark, rendering it
distinct from any other watermark in the same sample stream.
14. The method according to claim 13, further comprising the step
of: pre-processing sample windows in the sample stream to be
watermarked.
15. The method according to claim 14, further comprising the step
of: determining which sample windows will contain the individual
digital watermark to be encoded.
16. The method according to claim 15, further comprising the step
of: calculating a size of the independent information comprising
the digital watermark plus a size of an added hash value to
determine a number of sample windows required to contain a complete
watermark.
17. The method according to claim 16, further comprising the step
of: computing a secure one way hash function of the carrier signal
data in said sample windows, wherein said hash function is
insensitive to changes introduced into the carrier signal for the
purpose of carrying digital watermark information.
18. The method according to claim 17, further comprising the step
of: concatenating the hash value with the independent information,
creating an expanded, unique digital watermark.
19. The method according to claim 2, wherein the step of
steganographically encoding independent information into the
carrier signal causes an imperceptible change in the carrier
signal.
20. A method for steganographically protecting a digital signal
comprising the steps of: a) providing a carrier signal that has
been encoded with independent information; b) using a stega-cipher
to steganographically decode independent information including a
digital watermark from the carrier signal; c) selecting a mask set,
said mask set including one or more random or pseudo-random series
of bits, referred to as masks; d) selecting a random or
pseudo-random start of message delimiter; and e) selecting an input
sample stream to be decoded.
21. The method according to claim 20, wherein the carrier signal
includes a stream of digital samples.
22. The method according to claim 20, wherein the carrier signal
includes a continuous analog waveform.
23. The method according to claim 20, wherein the digital watermark
includes at least one selected from the group consisting of: rights
ownership identification, authorship identification of the encoded
carrier signal, ownership identification of a unique copy of the
encoded carrier signal, and a serialization code uniquely
identifying a copy of the encoded carrier signal.
24. The method according to claim 20, further comprising the step
of: decoding a single message bit from a single sample by reading a
single bit of the single sample as the message bit.
25. The method according to claim 20, further comprising the step
of: decoding a single message bit from a single sample by mapping
the single sample in the range of sample values which indicate a
particular message bit value.
26. The method according to claim 20, further comprising the step
of: decoding a single message bit from a single spectral value by
mapping the single spectral value into a range of sample values
which indicate a particular message bit value.
27. The method according to claim 20, further comprising the step
of: f) separating the input sample stream into smaller discrete
sample windows comprising segments of the input sample stream.
28. The method according to claim 27, further comprising the step
of: g) using positions within one of the sample windows and a
position within the input stream to index random or pseudo-random
masks and compute a mapping function to determine decoding
positions and to decode digital watermark information from the
sample window.
29. The method according to claim 28, further comprising the step
of: h) computing a spectral transform of the sample window prior to
digital watermark data decoding.
30. The method according to claim 28, further comprising the step
of: h) validating at least one selected from the group consisting
of: (1) a hash value computed on the start of message delimiter,
and (2) a digital signature of the start of message delimiter,
wherein the step h) of validating occurs after the start of message
delimiter and the encoded information of said hash value or said
digital signature have been decoded and the validation consists of
computing an appropriate result using the start of message
delimiter, comparing it to a value in the decoded data, and
verifying any signature.
31. The method according to claim 20, further comprising the step
of: obtaining a unique hash value contained in the independent
information comprising part of the digital watermark.
32. The method according to claim 31, further comprising the
additional step of: re-processing sample windows in the sample
stream which contained the decoded watermark.
33. The method according to claim 32, further comprising the step
of: computing a secure one way hash function of the carrier signal
data in said sample windows, wherein said hash function is
insensitive to changes introduced into the carrier signal for the
purpose of carrying digital watermark information.
34. The method according to claim 33, further comprising the step
of: comparing the computed hash value to the value contained in the
watermark.
35. The method according to claim 20, wherein the step of
steganographically decoding independent information into the
carrier signal causes an imperceptible change in the carrier
signal.
Description
DEFINITIONS
Several terms of art appear frequently in the following. For ease
of reference they are defined here as follows:
"Content" refers to multimedia content. This term encompasses the
various types of information to be processed in a multimedia
entertainment system. Content specifically refers to digitized
audio, video or still images in the context of this discussion.
This information may be contained within files on a multimedia
computer system, the files having a particular format specific to
the modality of the content (sound, images, moving pictures) or the
type of systems, computer or otherwise, used to process the
content.
"Digitized" refers to content composed of discrete digital samples
of an otherwise analog media, which approximate that media inside a
computer or other digital device. For instance, the sound of music
occurs naturally, and is experienced by humans as an analog
(continuous) sound wave. The sound can be digitized into a stream
of discrete samples, or numbers, each of which represents an
approximate value of the amplitude of the real analog wave at a
particular instant in time. These samples can be stored in files in
a computer and then used to recreate the original sound wave to a
high degree of accuracy. In general, content entering a digital
system is digitized by Analog to Digital converters (A/D) and
analog media are recreated by the digital system using a Digital to
Analog (D/A) converter. In the context of this discussion content
is always digitized content.
"Cryptography" is a field covering numerous techniques for
scrambling information conveying messages so that when the message
is conveyed between the sender and receiver an unintended party who
intercepts this message cannot read it, or extract useful
information from it.
A "Public Key Cryptosystem" is a particular cryptographic system
where all parties possess pairs of keys for encryption and
decryption. Parties to this type of system freely distribute their
public keys, which other may use to encrypt messages to the owner
of the public key. Such messages are decrypted by the receiver with
the private key. Private keys are never distributed. A message
encrypted with a public key can only be decrypted with the
corresponding private key, and vice versa. A message encrypted with
a private key is said to have been signed by the owner of that key.
Anyone in possession of the public key may decrypt the message and
know that it was encrypted, and thus signed, by the owner of the
public key, since only they possess the corresponding private
key.
"Steganography" is a field distinguished from cryptography, but
associated with it, that covers numerous methods for hiding an
informational message within some other medium, perhaps another
unrelated message, in such a manner that an unintended party who
intercepts the medium carrying die hidden message does not know it
contains this hidden message and therefore does not obtain the
information in the hidden message. In other words, steganography
seeks to hide messages in plain view.
BACKGROUND OF THE INVENTION
In the current environment of computer networks and the
proliferation of digital or digitized multimedia content which may
be distributed over such networks, a key issue is copyright
protection. Copyright protection is the ability to prevent or deter
the proliferation of unauthorized copies of copyrighted works. It
provides a reasonable guarantee that the author of a copyrighted
work will be paid for each copy of that work.
A fundamental problem in the digital world, as opposed to the world
of physical media, is that a unlimited number of perfect copies may
be made from any piece of digital or digitized content. A perfect
copy means that if the original is comprised of a given stream of
numbers, then the copy matches the original, exactly, for each
number in the stream. Thus, there is no degradation of the original
signal during the copy operation. In an analog copy, random noise
is always introduced, degrading the copied signal.
The act of making unlicensed copies of some content, digital or
analog, whether audio, video, software or other, is generally known
as piracy. Piracy has been committed for the purpose of either
profit from the sale of such unlicensed copies, or to procure for
the "pirate" a copy of the content for personal use without having
paid for it.
The problem of piracy has been made much worse for any type of
content by the digitization of content. Once content enters the
digital domain, an unlimited number of copies may be made without
any degradation, if a pirate finds a way to break whatever
protection scheme was established to guard against such abuses, if
any. In the analog world, there is generally a degradation in the
content (signal) with each successive copy, imposing a sort of
natural limit on volume of piracy.
To date, three general types of schemes have been implemented in an
attempt to protect copyrights.
1) Encryption
2) Copy Protection
3) Content Extensions
Copy Protection and Content Extensions generally apply in the
digital world only, while a scheme related to Encryption, commonly
known as scrambling, my be applied to an analog signal. This is
typical in analog cable systems.
Encryption scrambles the content. Before the content is made ready
for delivery, whether on floppy disk, or over a network, it must be
encrypted, or scrambled. Once the content has been encrypted, it
cannot be used until it is decrypted, or unscrambled. Encrypted
audio data might sound like incomprehensible screeching, while an
encrypted picture or video might appear as random patterns on a
screen. The principle of encryption is that you are free to make as
many copies as you want, but you can't read anything that makes
sense until you use a special key to decrypt, and you can only
obtain the key by paying for the content.
Encryption has two problems, however, 1) Pirates have historically
found ways to crack encryption, in effect, obtaining the key
without having paid for it; and 2) Once a single legitimate copy of
some content has been decrypted, a pirate is now free to make
unlimited copies of the decrypted copy. In effect, in order to sell
an unlimited quantity of an encrypted piece of software, the pirate
could simply buy one copy, which they are entitled to decrypt.
Copy Protection includes various methods by which a software
engineer can write the software in a clever manner to determine if
it has been copied, and if so to deactivate itself. Also included
are undocumented changes to the storage format of the content. Copy
protection was generally abandoned by the software industry, since
pirates were generally just as clever as the software engineers and
figured out ways to modify their software and deactivate the
protection. The cost of developing such protection was not
justified considering the level of piracy which occurred despite
the copy protection.
Content Extension refers to any system which attaches some extra
information to the original content which indicates whether or not
a copy may be made. A software or hardware system must be
specifically built around this scheme to recognize the additional
information and interpret it in an appropriate manner. An example
of such a system is the Serial Copyright Management System embedded
in Digital Audio Tape (DAT) hardware. Under this system, additional
information is stored on the disc immediately preceding each track
of audio content which indicates whether or not it can be copied.
The hardware reads this information and uses it accordingly.
A fundamental problem with Encryption and Content Extension is the
"rogue engineer". An employee who helped design such a system or an
individual with the knowledge and means to analyze such a system
can modify it to ignore the copyright information altogether, and
make unlicensed copies of the content. Cable piracy is quite
common, aided by illicit decoder devices built by those who
understand the technical details of the cable encryption system.
Although the cable systems in question were actually based on
analog RF signals, the same principle applies to digital
systems.
The practical considerations of weak encryption schemes and rogue
engineers have served to limit the faith which may be put in such
copyright protection schemes. The invention disclosed herein serves
to address these problems with conventional systems for digital
distribution. It provides a way to enforce copyright online. The
invention draws on techniques from two fields, cryptography, the
art of scrambling messages so that only the intended recipient may
read them, and steganography, a term applied to various techniques
for obscuring messages so that only the intended parties to a
message even know that a message has been sent, thus it is termed
herein as a stega-cipher. The stega-cipher is so named because it
uses the steganographic technique of hiding a message in multimedia
content, in combination with multiple keys, a concept originating
in cryptography. However, instead of using the keys to encrypt the
content, the stega-cipher uses these keys to locate the hidden
message within the content. The message itself is encrypted which
serves to further protect the message, verify the validity of the
message, and redistribute the information in a random manner so
that anyone attempting to locate the message without the keys
cannot rely on pre-supposed knowledge of the message contents as a
help in locating it.
SUMMARY OF THE INVENTION
The invention disclosed herein combines two techniques,
steganography--obscuring information that is otherwise in plain
sight, and cryptography--scrambling, information that must be sent
over unsecured means, in a manner such that only the intended
recipient may success fully unscramble it. The net effect of this
system is to specifically watermark a piece of content so that if
it is copied, it is possible to determine who owned the original
from which the copies were made, and hence determine responsibility
for the copies. It is also a feature of the system to uniquely
identify the content to which it is applied.
For a comprehensive discussion of cryptography, its theory,
applications and specific algorithms, see APPLIED CRYPTOGRAPHY, by
Bruce Schneier, which is herein incorporated by reference at pages
66-68, 387-392.
Steganography is discussed briefly in THE CODE BREAKERS by David
Kahn, which is herein incorporated by reference at pages xiii,
81-83, 522-526, and 873. An example application, Stego by Romana
Machado, is also available for the Apple Macintosh. Stego can be
found at the internet uniform resource locator
"ftp://sumex-aim.stanford.edu/info-mac/cmp/stego10a2.hqx". This
application demonstrates a simple steganographic technique to
encode a text message into a graphical image without significantly
distorting the image.
The invention improves upon the prior art by providing a manner for
protecting copyright in the digital domain, which neither
steganography or cryptography does. It improves specifically on
steganography by making use of special keys which dictate exactly
where within a larger chunk of content a message is to be hidden,
and makes the task of extracting such a message without the proper
key the equivalent of looking for a needle in a haystack.
The information encoded by the Stega-Cipher process serves as a
watermark which identifies individual copies of content legally
licensed to specific parties. It is integral with the content. It
cannot be removed by omission in a transmission. It does not add
any overhead to signal transmission or storage. It does allow the
content to be stored to and used with traditional offline analog
and digital media, without modification or significant signal
degradation. These aspects of the stega-cipher all represent
improvements to the art.
That is, it forces would-be pirates to damage the content in order
to guarantee the disabling of the watermark.
The invention described herein is used for protecting and enforcing
copyrights in the digital or on-line domain, where there are no
physical limitations on copying copyrighted content.
The invention uniquely identifies every copy of multimedia content
made using the invention, composed of digitized samples whether
compressed or uncompressed, including but not limited to still
digital images, digital audio, and digital video.
The invention is for use in meterware or pay-by-use systems where
an online user incurs a charge each time they access a particular
piece of content, or uses a software title.
The invention is for use as a general improvement to cryptographic
techniques to increase the complexity of crytanalysis on a given
cipher.
It is considered that the method and steps of the present invention
will be modified to account for the effects of lossy compression
schemes on the samples and particularly includes modification to
handle MPEG compressed audio and video.
It is considered that statistical data spreading and recovery
techniques, error coding or spread spectrum processing techniques
might be applied in the invention to handle the effects of lossy
compression, or counter the effects of a randomization attack.
It is considered that the apparatus described might be further
specialized and optimized in hardware by replacing general purpose
data buses and CPU or DSP driven operations with hardwired
circuitry, incorporated in one or more special purpose ICs.
It is considered that the apparatus will be modeled and implemented
in software on general purpose computer platforms.
It is considered that stega-cipher hardware could be embedded in a
consumer electronics device and used to not only identify content
and copyright, but to enable use of that content.
DETAILED DESCRIPTION
I. Digital Copyright Stega-Cipher Protocol and the Decode/Encode
Program
The purpose of the program described here is to watermark digital
multimedia content for distribution to consumers through online
services in such a way as to meet the following criteria.
Given a unique piece of multimedia content, composed of digitized
samples, it is desirable to:
1) Uniquely identify this particular piece of content from others
in a manner which is secure and undeniable (e.g. to know whether a
digital audio recording is "My Way" by Frank Sinatra, or "Stairway
to Heaven", by Led Zeppelin), and in a manner such that this
identification can be performed automatically by an electronic
device or mechanism. 2.) Uniquely identify the copyright owner of
the content, and the terms under which it may be distributed in
general, in a manner which is secure and undeniable. 3) At such
time as is necessary, additionally, uniquely identify in a secure
and undeniable manner the licensed publisher who received a
particular copy of the content, and the terms under which they may
redistribute or resell it. 4) At such time as is necessary,
additionally, uniquely identify in a secure and undeniable manner,
the licensed subscriber who received a particular copy of the
content from the publisher described in item 3.
The program described in more detail below combines the techniques
of cryptography and steganography to hide a securely encrypted
digital copyright certificate which contains information satisfying
the criteria listed above, in such a manner as to be integral with
the content, like a watermark on paper, so that possession of the
content dictates possession of the watermark information. In
addition, the watermark cannot be "found" or successfully decoded,
without possession of the correct "masks" or keys, available only
to those legitimately authorized, namely, those parties to a
commercial transaction involving the sale of a copy of the content.
Finally, the ability to distribute such watermarked content in a
system which implements the watermark scheme is denied without a
successfully decoded watermark. Because well known and tested
cryptographic techniques are used to protect the certificate
itself, these certificates are virtually impossible to forge.
Finally, the watermark cannot be erased without significantly
damaging the content.
The basic program represents a key part of the invention itself.
This program is then used as the method by which copyright
information is to be associated in an integral manner with the
content. This is a concept absent from copy protection, encryption
and content extension schemes. The copyright information itself can
be made undeniable and unforgeable using cryptographic techniques,
so that through it an audit trail of ownership my be established
for each copy of a given piece of content, thus customizing each
copy to a particular owner, in a way that can be used to identify
the owner.
The value of the stega-cipher is that it provides a way to
watermark the content in a way that changes it slightly, but does
not impact human perception significantly. And, furthermore, that
it is made difficult to defeat since one must know exactly where
the information resides to extract it for analysis and use in
forgery attempts, or to remove it without overly degrading the
signal. And, to try to forge copyright information one must first
be able to analyze the encrypted copyright information, and in
order to do that, one must be able to find it, which requires
masks.
II. Example Embodiment of General Processing
Digital audio data is represented by a series of samples in 1
dimension, {S.sub.1, S.sub.2, S.sub.3 . . . S.sub.n}
This series is also referred to as a sample stream. The sample
stream approximates an analog waveform of sound amplitude over
time. Each sample represents an estimate of the wave amplitude at
the instant of time the sample is recorded. For monaural audio,
there is one such sample stream. Stereo audio is comprised of two
sample streams, one representing the right channel, and the other
representing the left. Each stream is used to drive a corresponding
speaker to reproduce the stereo sound.
What is referred to as CD quality audio is characterized by 16 bit
(2 byte) stereo samples, recorded at 44.1 Khz, or 44,100 samples
per second in each channel. The dynamic range of sound reproduction
is directly proportional to the number of bits per sample. Some
lower quality recordings are done at 8 bits. A CD audio recording
can be stored using any scheme for containing the 2 sample streams
in their entirety. When these streams are played back at the same
frequency they were recorded at, the sound recorded is reproduced
to a high degree of accuracy.
The sample stream is processed in order from first sample to last.
For the purpose of the invention disclosed, the stream is separated
into sample windows, each of which has a fixed number of
consecutive samples from the stream, and where windows do not
overlap in the sample stream. Windows may be contiguous in the
sample stream. In this discussion assume each window contains 128
samples, and that windows are contiguous. So, the windows within
the stream look like {[S.sub.1, S.sub.2, S.sub.3 . . . S.sub.128],
[S.sub.129, S.sub.130, S.sub.131 . . . S.sub.256], . . .
[S.sub.n-128 . . . S.sub.n]} where [ . . . ] denotes each window
and any odd samples at the end of the stream which do not
completely fill a window can be ignored, and simply passed through
the system unmodified.
These windows will be used as input for the discrete Fast Fourier
Transform (and its inverse) operation.
Briefly, Fourier Transform methods are based on the principle that
a complex waveform, expressed as amplitude over time and
represented by a sample stream, is really the sum of a number of
simple waveforms, each of which oscillate at different
frequencies.
By complex, it is meant that the value of the next sample is not
easily predicted from the values of the last N samples or the time
of the sample. By simple it is meant that the value of the sample
is easily predictable from the values of the last N samples and/or
the time of the sample.
The sum of multiple simple waves is equivalent to the complex wave.
The discrete FFT and its inverse simply translate a limited amount
of data from one side of this equivalence to the other, between the
complex waveform and the sum of simple waves. The discrete FFT can
be used to translate a series of samples representing amplitude
over time (the complex wave, representing a digital audio
recording) into the same number of samples representing total
spectral energy in a given range of frequencies (the simple wave
components) at a particular instant of time. This instant is the
time in the middle of the original amplitude/time samples. The
inverse discrete FFT translates the data in the other direction,
producing the complex waveform, from its simpler parts.
Each 128 sample window will be used as an input to the discrete
FFT, resulting in 128 bins representing each of 128 frequency
bands, ranging from 0 Hz to 22 Khz (the Nyquist frequency, or 1/2
the sampling rate).
Information can be encoded into the audio signal in the frequency
domain or in the time domain. In the latter case, no FFT or inverse
FFT is necessary. However, encoding in the frequency domain is
recommended, since its effects are scattered over the resultant
time domain samples, and not easily predicted. In addition,
frequency domain encoding makes it more likely that randomization
will result in noticeable artifacts in the resultant signal, and
therefore makes the stega-cipher more defensible against such
attacks. It is in the frequency domain that additional information
will be encoded into the audio signal for the purpose of this
discussion. Each frequency band in a given time slice can
potentially be used to store a small portion of some additional
information to be added to the signal. Since these are discrete
estimates, there is some room for error which will not
significantly effect the perceived quality of the signal,
reproduced after modification, by the inverse FFT operation. In
effect, intentional changes, which cannot be distinguished from
random variations are introduced in the frequency domain, for the
purpose of storing additional information in the sample stream.
These changes are minimized so as not to adversely affect the
perceived quality of the reproduced audio signal, after it has been
encoded with additional information in the manner described below.
In addition, the location of each of these chances is made
virtually impossible to predict, an innovation which distinguishes
this scheme from simple steganographic techniques.
Note that this process differs from the Nagata, et al. U.S. Pat.
Nos. 4,979,210 and 5,073,925, which encode information by
modulating an audio signal in amplitude/time domain. It also
differs in that the modulations introduced in the Nagata process
(which are at very low amplitude and frequency relative to the
carrier wave as to remain inaudible) carry only copy/don't copy
information, which is easily found and circumvented by one skilled
in the art. Also, there is no limitation in the stega-cipher
process as to what type of information can be encoded into the
signal, and there is more information storage capacity, since the
encoding process is not bound by any particular frequency of
modulation but rather by the number of samples available. The
granularity of encoding in the stega-cipher is determined by the
sample window size, with potentially 1 bit of space per sample or
128 bits per window (a secure implementation will halve this to 64
bits). In Nagata, et al. the granularity of encoding is fixed by
the amplitude and frequency modulation limits required to maintain
inaudibility. These limits are relatively low, and therefore make
it impractical to encode more than simple copy/don't copy
information using the Nagata process.
III. Example Embodiment of Encoding and Decoding
A modification to standard steganographic technique is applied in
the frequency domain described above, in order to encode additional
information into the audio signal.
In a scheme adapted from cryptographic techniques, 2 keys are used
in the actual encode and decode process. For the purposes of this
invention the keys are referred to as masks. One mask, the primary,
is applied to the frequency axis of FFT results, the other mask is
applied to the time axis (this will be called the convolution
mask). The number of bits comprising the primary mask are equal to
the sample window size in samples (or the number of frequency bands
computed by the FFT process), 128 in this discussion. The number of
bits in the convolution mask are entirely arbitrary. This
implementation will assume a time mask of 1024 bits. Generally the
larger the key, the more difficult it is to guess.
Prior to encoding, the primary and convolution masks described
above are generated by a cryptographically secure random generation
process. It is possible to use a block cipher like DES in
combination with a sufficiently pseudo-random seed value to emulate
a cryptographically secure random bit generator. These keys will be
saved along with information matching them to the sample stream in
question in a database for use in decoding, should that step become
necessary.
Prior to encoding, some additional information to be encoded into
the signal is prepared and made available to the encoder, in a bit
addressable manner (so that it may be read one bit at a time). If
the size of the sample stream is known and the efficiency
characteristics of the stega-cipher implementation are taken into
account, a known limit may be imposed on the amount of this
additional information.
The encoder captures one sample window at a time from the sample
stream, in sequential, contiguous order. The encoder tracks the
sequential number of each window it acquires. The first window is
0. When the number of windows processed reaches the number of bits
in the window mask, minus one, the next value of the window counter
will be reset to 0.
This counter is the convolution index or phase. In the current
implementation it is used as a simple index into the convolution
bitmask. In anticipated developments it will be used to perform
convolution operations on the convolution mask to determine which
bit to use. For instance the mask might by rotated by a number
corresponding to the phase, in bits to the left and XORed with the
primary mask to produce a new mask, which is then indexed by the
phase. There are many possibilities for convolution.
The encoder computes the discrete FFT of the sample window.
Starting with the lowest frequency band, the encoder proceeds
through each band to the highest, visiting each of the 128
frequency bands in order. At each band value, the encoder takes the
bit of the primary mask corresponding to the frequency band in
question, the bit of the convolution mask corresponding to the
window in question, and passes these values into a boolean
function. This function is designed so that it has a near perfectly
random output distribution. It will return true for approximately
50% of its input permutations, and false for the other 50%. The
value returned for a given set of inputs is fixed, however, so that
it will always return the same value given the same set of
inputs.
If the function returns true, the current frequency band in the
current window is used in the encoding process, and represents a
valid piece of the additional information encoded in the signal. If
the function returns false, this cell, as the frequency band in a
given window is called, is ignored in the process. In this manner
it is made extremely difficult to extract the encoded information
from the signal without the use of the exact masks used in the
encoding process. This is one place in which the stega-cipher
process departs from traditional steganographic implementations,
which offer a trivial decode opportunity if one knows the
information is present. While this increases the information
storage capacity of the carrier signal, it makes decoding trivial,
and further degrades the signal. Note that it is possible and
desirable to modify the boolean cell flag function so that it
returns true <50% of the time. In general, the fewer cells
actually used in the encode, the more difficult they will be to
find and the less degradation of content will be caused, provided
the function is designed correctly. There is an obvious tradeoff in
storage capacity for this increased security and quality.
The encoder proceeds in this manner until a complete copy of the
additional information has been encoded in the carrier signal. It
will be desirable to have the encoder encode multiple copies of the
additional information continuously over the duration of the
carrier signal, so that a complete instance of this information may
be recovered from a smaller segment of a larger signal which has
been split into discontinuous pieces or otherwise edited. It is
therefore desirable to minimize the size of the information to be
encoded using both compact design and pre-encoding compression,
thus maximizing redundant encoding, and recoverability from smaller
segments. In a practical implementation of this system it is likely
the information will be first compressed by a known method, and
then encrypted using public-key techniques, before being encoded
into the carrier signal.
The encoder will also prepare the package of additional information
so that it contains an easily recognizable start of message
delimeter, which can be unique to each encoding and stored along
with the keys, to serve as a synchronization signal to a decoder.
The detection of this delimeter in a decoding window signifies that
the decoder can be reasonably sure it is aligned to the sample
stream correctly and can proceed in a methodic window by window
manner. These delimeters will require a number of bits which
minimizes the probability that this bit sequence is not reproduced
in a random occurrence, causing an accidental misalignment of the
decoder. A minimum of 256 bits is recommended. In the current
implementation 1024 bits representing a start of message delimeter
are used. If each sample is random, then each bit has a 50%
probably of matching the delimeter and the conditional probability
of a random match would be 1/2.sup.1024. In practice, the samples
are probably somewhat less than random, increasing the probability
of a match somewhat.
The decode process uses the same masks in the same manner, only in
this case the information is extracted one bit at a time from the
carrier signal.
The decoder is assumed to have access to the proper masks used to
encode the information originally. These masks might be present in
a database, which can be indexed by a value, or values computed
from the original content, in a manner insensitive to the
modifications to the content caused by the stega-cipher process.
So, given an arbitrary piece of content, a decoder might first
process the content to generate certain key values, and then
retrieve the decode masks associated with the matching key values
from the database. In the case where multiple matches occur, or
none are found, it is conceivable that all mask sets in the
database could be tried sequentially until a valid decode is
achieved, or not, indicating no information is present.
In the application of this process, it is anticipated that encoding
operations may be done on a given piece of content up to 3 times,
each adding new information and using new masks, over a sub-segment
of the content, and that decode operations will be done
infrequently. It is anticipated that should it become necessary to
do a search of a large number of masks to find a valid decode, that
this process can be optimized using a guessing technique based on
close key matching, and that it is not a time critical application,
so it will be feasible to test large numbers of potential masks for
validity on a given piece of content, even if such a process takes
days or weeks on powerful computers to do a comprehensive search of
known mask sets.
The decode process is slightly different in the following respect.
Whereas the encoding process can start at any arbitrary point in
the sample stream, the decode process does not know where the
encode process began (the exact offset in samples to the start of
the first window). Even though the encode process, by convention,
starts with sample 0, there is no guarantee that the sample stream
has not been edited since encoding, leaving a partial window at the
start of the sample stream, and thus requiring the decoder to find
the first complete window to start the decode. Therefore, the
decode process will start at the first sample, and shift the sample
window along by 1 sample, keeping the window index at 0, until it
can find a valid decode delimeter encoded in the window. At this
point, the decoder knows it has synchronized to the encoder, and
can then proceed to process contiguous windows in a more expedient
manner.
Example Calculations based on the described implementation for
adding copyright certificate information to CD quality digital
audio:
In a stream of samples, every 128 samples will contain, on average
64 bits of certificate related information. Digital audio is
composed of 16 bit samples, at 44.1 Khz, or 44,100 samples per
second. Stereo audio provides 2 streams of information at this
rate, left and right, or 88,200 samples per second. That yields
approximately 689 contiguous sample windows (of 128 samples) per
second in which to encode information. Assume a song is 4 minutes
long, or 240 seconds. This yields 240*689=165,360 windows, which on
average (50% utilization) contain 64 bits (8 bytes) each of
certificate information. This in turns gives approximately 1291 Kb
of information storage space per 4 minute stereo song (1.2 MB).
There is ample room for redundant encoding of information
continuously over the length of the content. Encoding 8 bytes for
every 256 bytes represents 3.1% of the signal information. Assuming
that a copyright certificate requires at most approximately 2048
bytes (2K), we can encode the same certificate in 645 distinct
locations within the recording, or approximately every 37/100ths of
a second.
Now to account for delimeters and synchronization information.
Assuming a sync marker of 1024 bits to avoid random matches, then
we could prefix each 2K certificate block with this 1024 bit
marker. It takes 256 windows to store 2K, and under this proposed
scheme, the first 16 windows are reserved for the sync marker. A
decoder could search for this marker by progressively matching each
of the first 16 windows (64 bits at a time) against the
corresponding portion of the sync marker. The decoder could reset
the match advancing through the sample stream, as soon as one
window did not conform to the sync marker, and proceed in this
manner until it matches 16 consecutive windows to the marker, at
which point it is synchronized.
Under this scheme, 240 windows, or 1.92K remain for storing
certificate information, which is not unreasonable.
IV. Possible Problems, Attacks and Subsequent Defenses
A. Randomization
The attacker simply randomizes the least significant bits of each
data point in the transform buffer, obliterating the
synchronization signal and the watermark. While this attack can
remove the watermark, in the context in which stega-cipher is to be
used, the problem of piracy is kept to a minimum at least equal to
that afforded by traditional media, since the system will not allow
an unwatermarked piece of content to be traded for profit and
watermarks cannot be forged without the proper keys, which are
computationally difficult to obtain by brute-force or crytanalysis.
In addition, if the encoding is managed in such a way as to
maximize the level of changes to the sample stream to be just at
the threshold below human perception, and the scheme is implemented
to anticipate randomization attempts, it is possible to force the
randomization level to exceed the level that can be perceived and
create destructive artifacts in the signal, in much the same manner
as a VHS cassette can be manufactured at a minimal signal level, so
that a single copy results in unwatchable static.
B. Low Bit-Depth Bitmaps (Black & White Images)
These bitmaps would be too sensitive to the steganization process,
resulting in unacceptable signal degradation, and so are not good
candidates for the stega-cipher process. The problem may be
circumvented by inflating bit-depth, although this is an
inefficient use of space and bandwidth.
C. Non-Integer Transforms
The FFT is used to generate spectral energy information for a given
audio signal. This information is not usually in integer format.
Computers use methods of approximation in these cases to represent
the real numbers (whole numbers plus fractional amounts). Depending
on the exact value of the number to be represented slight errors,
produced by rounding off the nearest real number that can be
completely specified by the computer occur. This will produce some
randomization in the least significant bit or bits. In other words,
the same operation on the same sample window might yield slightly
different transform values each time. It is possible to circumvent
this problem using a modification to the simple LSB steganographic
technique described later. Instead of looking at the LSB, the
stega-cipher can use an energy quantization technique in place of
the LSB method. Some variant of rounding the spectral energy values
up or down, with a granularity greater than the rounding error
should work, without significantly degrading the output
samples.
V. A Method and Protocol for Using the Stega-Cipher
The apparatus described in the claims below operates on a window by
window basis over the sample stream. It has no knowledge of the
nature of the specific message to be encoded. It merely indexes
into a bit stream, and encodes as many of those bits as possible
into a given sample window, using a map determined by the given
masks.
The value of encoding information into a single window in the
sample stream using such an apparatus may not be inherently
apparent until one examines the manner in which such information
will be used. The protocol discussed in this section details how
messages which exceed the encoding capacity of a single sample
window (128 samples) may be assembled from smaller pieces encoded
in the individual windows and used to defend copyrights in an
online situation.
An average of 64 bits can be encoded into each window, which equals
only 8 bytes. Messages larger than 8 bytes can be encoded by simply
dividing the messages up and encoding small portions into a string
of consecutive windows in the sample stream. Since the keys
determine exactly how many bits will be encoded per window, and an
element of randomness is desirable, as opposed to perfect
predictability, one cannot be certain exactly how many bits are
encoded into each window.
The start of each message is marked by a special start of message
delimeter, which, as discussed above is 1024 bits, or 128 bytes.
Therefore, if precisely 8 bytes are encoded per window, the first
16 windows of any useable message in the system described here are
reserved for the start of message delimeter. For the encoder, this
scheme presents little challenge. It simply designates the first
sample window in the stream to be window 0, and proceeds to encode
the message delimeter, bit-by-bit into each consecutive window. As
soon as it has processed the last bit of the SOM delimeter it
continues by encoding 32 bits representing the size, in bytes of
the complete message to follow. Once the 32nd and final bit of the
size is encoded, the mess age itself is encoded into each
consecutive window, one bit at a time. Some windows may contain
more encoded bits then others, as dictated by the masks. As the
encoder processes each window in the content it increments its
window counter. It uses this counter to index into the window mask.
If the number of windows required to encode a complete message is
greater than the size of this mask, 256 bits in this case, or 256
windows, then it simply resets the counter after window 255, and so
on, until a complete message is encoded. It can then start over, or
start on a new message.
The decoder has a bigger challenge to face. The decoder is given a
set of masks, just like encoder. Unlike the encoder, the decoder
cannot be sure that the first series of 128 samples it receives are
the window 0 start of message, encoded by the decoder. The sample
stream originally produced by an encoder may have been edited by
clipping its ends randomly or splicing pieces together. In that
case, the particular copy of the message that was clipped is
unrecoverable. The decoder has the start of message delimeter used
to encode the message that the decoder is looking for. In the
initial state, the decoder assumes the first window it gets is
window 0. It then decodes the proper number of bits dictated by the
masks it was given. It compares these bits to the corresponding
bits of the start of message delimeter. If they match, the decoder
assumes it is still aligned, increments the window counter and
continues. If the bits do not match, the decoder knows it is not
aligned. In this case, it shifts one more sample onto the end of
the sample buffer, discarding the first sample, and starts over.
The window counter is set to 0. The decoder searches one sample at
a time for an alignment lock. The decoder proceeds in this manner
until it has decoded a complete match to the start of message
delimeter or it exhausts the sample stream without decoding a
message. If the decoder can match completely the start of message
delimeter bit sequence, it switches into aligned mode. The decoder
will now advance through the sample stream a full window at a time
(128 samples). It proceeds until it has the 32 bits specifying the
message size. This generally won't occupy more than 1 complete
window. When the decoder has locked onto the start of message
delimeter and decoded the message size, it can now proceed to
decode as many consecutive additional windows as necessary until it
has decoded a complete message. Once it has decoded a complete
message, the state of the decoder can be reset to un-synchronized
and the entire process can be repeated starting with the next 128
sample window. In this manner it is not absolutely necessary that
encoding windows be contiguous in the sample stream. The decoder is
capable of handling random intervals between the end of one message
and the start of another.
It is important to note that the circuit for encoding and decoding
a sample window does not need to be aware of the nature of the
message, or of any structure beyond the start of message delimeter
and message size. It only needs to consider a single sample window,
its own state (whether the decoder is misaligned, synchronizing, or
synchronized) and what bits to encode/decode.
Given that the stega-cipher apparatus allows for the encoding and
decoding of arbitrary messages in this manner, how can it be used
to protect copyrights?
The most important aspect of the stega-cipher in this respect is
that fact that it makes the message integral with the content, and
difficult to remove. So it cannot be eliminated simply by removing
certain information prepended or appended to the sample stream
itself. In fact, removing an arbitrary chunk of samples will not
generally defeat the stega-cipher either.
Given that some information can be thus integrated with the content
itself, the question is then how best to take advantage of this
arrangement in order to protect copyrights.
The following protocol details how the stega-cipher will be
exploited to protect copyrights in the digital domain.
In a transaction involving the transfer of digitized content, there
are at least 3 functions involved:
The Authority is a trusted arbitrator between the two other
functions listed below, representing parties who actually engage in
the transfer of the content. The Authority maintains a database
containing information on the particular piece of content itself
and who the two parties engaged in transferring the content are.
The Authority can perform stega-cipher encoding and decoding on
content.
The Publisher, or online distributor is the entity which is sending
the copyrighted content to another party. The Publisher can perform
stega-cipher encoding and decoding on content.
The Consumer is the person or entity receiving the copyrighted
content, generally in exchange for some consideration such as
money. The consumer cannot generally perform stega-cipher encoding
or decoding on content.
Each of these parties can participate in a message exchange
protocol using well known public-key cryptographic techniques. For
instance, a system licensing RSA public key algorithms might be
used for signed and encrypted message exchange. This means that
each party maintains a public key/private key pair, and that the
public keys of each party are freely available to any other party.
Generally, the Authority communicates via electronic links directly
only to the Publisher and the Consumer communicates directly only
with the publisher.
Below is an example of how the protocol operates from the time a
piece of content enters an electronic distribution system to the
time it is delivered to a Consumer.
A copyright holder (an independent artist, music publisher, movie
studio, etc.) wishes to retail a particular title online. For
instance, Sire Records Company might wish to distribute the latest
single from Seal, one of their musical artists, online. Sire
delivers a master copy of this single, "Prayer for the Dying", to
the Authority, Ethical Inc. Ethical converts the title into a
format suitable for electronic distribution. This may involve
digitizing an analog recording. The title has now become content in
the context of this online distribution system. The title is not
yet available to anyone except Ethical Inc., and has not yet been
encoded with the stega-cipher watermark. Ethical generates a Title
Identification and Authentication (TIA) certificate. The
certificate could be in any format. In this example it is a short
text file, readable with a small word-processing program, which
contains information identifying the title the artist the copyright
holder the body to which royalties should be paid general terms for
publishers' distribution any other information helpful in
identifying this content
Ethical then signs the TIA with its own private key, and encrypts
the TIA certificate plus its signature with its own public key.
Thus, the Ethical can decrypt the TIA certificate at a later time
and know that it generated the message and that the contents of the
message have not been changed since generation.
Sire Records, which ultimately controls distribution of the
content, communicates to the Ethical a specific online Publisher
that is to have the right of distribution of this content. For
instance, Joe's Online Emporium. The Authority, Ethical Inc. can
transmit a short agreement, the Distribution Agreement to the
Publisher, Joe's Online Emporium which lists the content title the
publisher's identification the terms of distribution any
consideration paid for the right to distribute the content a brief
statement of agreement with all terms listed above
The Publisher receives this agreement, and signs it using its
private key. Thus, any party with access to the Joe's Online
Emporium's public key could verify that the Joe's signed the
agreement, and that the agreement has not been changed since Joe's
signed it. The Publisher transmits the signed Distribution
Agreement to the Authority, Ethical Inc.
Ethical Inc. now combines the signed TIA certificate and the
Distribution Agreement into a single message, and signs the entire
message using its private key. Ethical has now created a Publisher
Identification message to go into its own stega-cipher channel in
the content. Ethical Inc. now generates new stega-cipher masks and
encodes this message into a copy of the content using a
stega-cipher encoder. The Authority saves the masks as a Receipt in
a database, along with information on the details of the transfer,
including the title, artist and publisher.
Ethical then transfers this watermarked copy to the Joe's Online
Emporium, the Publisher. Well known encryption methods could be
used to protect the transfer between the Authority and the
Publisher. The Authority may now destroy its copy, which the
Publisher has received. The Publisher, Joe's Online Emporium now
assumes responsibility for any copies made to its version of the
content, which is a Publisher Master copy.
Finally, the Consumer, John Q. Public wishes to purchase a copy of
the content from Joe's Online Emporium. Joe's Emporium sends the
John Q. Public a short agreement via an electronic communication
link, similar to Publisher's Distribution Agreement, only this is a
Purchase Agreement, which lists the content title consumer
identification the terms of distribution the consideration pas for
the content a brief statement of agreement with the terms above
John Q. Public signs this agreement with his private key and
returns it to the Joe's Online Emporium. The Publisher, Joe's
prepares to encode its own stega-cipher watermark onto a copy of
the content by generating a set of masks for the algorithm. Joe's
Online Emporium then stores these masks (a receipt) in its own
database, indexed by title and consumer. Joe's Online Emporium
signs the agreement received from John Q. Public with the
Emporium's own private key, and forwards it to the Authority,
Ethical Inc., along with a copy of the masks. It is important to
note that this communication should be done over a secured channel.
The Authority verifies the Publisher and Consumer information and
adds its own signature to the end of the message, approving the
transaction, creating a Contract of Sale. The Authority adds the
Publisher's receipt (mask set) to its database, indexed by the
title, the publisher, and the consumer identification. The
Authority signs the Contract of Sale by encrypting it with their
private key. So anyone with the Authority's public key (any
Publisher) could decrypt the Contract of Sale and verify it, once
it was extracted from the content. The Publisher then transmits the
signed Contract of Sale back to the Publisher, who uses a
stega-cipher device to imprint this Contract as its own watermark
over the content. The Publisher then transmits the newly
watermarked copy to the Consumer, who is accepting responsibility
for it. The Publisher destroys their version of the consumer's
copy.
If this procedure is followed for all content distribution within
such an online system then it should be possible for the Authority
to identify the owner of a piece of content which appears to be
unauthorized. The Authority could simply try its database of
stega-cipher keys to decode the watermark in the content in
question. For instance, if a copy of Seal's latest single
originally distributed with stega-cipher watermarks showed up on an
Internet ftp site the Authority should be able to extract a TIA
Certificate and Distribution Agreement or a Contract of Sale
identifying the responsible party. If a Publisher sold this
particular copy to a Consumer, that particular publisher should be
able to extract a Contract of Sale, which places responsibility
with the Consumer. This is not a time critical application, so even
if it takes days or weeks, it is still worthwhile.
In a modification to the protocol discussed above, each Publisher
might act as its own Authority. However, in the context of online
services, this could open avenues of fraud committed by the
collusion of certain Publishers and Consumers. Using an Authority,
or one of several available Authorities to keep records of
Publisher-Consumer transactions and verify their details decreases
the likelihood of such events.
It should also be obvious that a similar watermarking system could
be used by an individual entity to watermark its own content for
its own purposes, weather online or in physical media. For
instance, a CD manufacturer could incorporate unique stega-cipher
watermarks into specific batches of its compact discs to indentify
the source of a pirate ring, or to identify unauthorized digital
copies made from its discs. This is possible because the
stega-cipher encoding works with the existing formats of digital
samples and does not add any new structures to the sample data that
cannot be handled on electronic or mechanical systems which predate
the stega-cipher.
VI. Increasing Confidence in the Stega-Cipher
The addition of a special pre-encoding process can make
stega-cipher certificates even more secure and undeniable. Hash
values may be incorporated which match exactly the content
containing the watermark to the message in the watermark itself.
This allows us a verification that the watermark decoded was
encoded by whomever signed it into this precise location in this
specific content.
Suppose one wants to use a 256 bit (32 byte) hash value which is
calculated with a secure one-way hash function over each sample in
each sample window that will contain the message. The hash starts
with a seed value, and each sample that would be processed by the
encoder when encoding the message is incorporated into the hash as
it is processed. The result is a 256 bit number one can be highly
confident is unique, or sufficiently rare to make intentionally
duplicating it with another series of samples difficult.
It is important that the hash function be insensitive to any
changes in the samples induced by the stega-cipher itself. For
instance, one might ignore the least significant bit of each sample
when computing the hash function, if the stega-cipher was
implemented using a least significant bit encode mode.
Based on the size of the non-hash message, one knows the
hash-inclusive message requires 32 more bytes of space. One can now
calculate the size of a signed encrypted copy of this message by
signing and encrypting exactly as many random bytes as are in the
message, and measuring the size of the output in bytes. One now
knows the size of the message to be encoded. One can pre-process
the sample stream as follows.
Proceed through the stega-cipher encode loop as described above.
Instead of encoding, however, calculate hash values for each window
series which will contain the message, as each sample is processed.
At the end of each instance of "encoding" take the resultant hash
value and use it to create a unique copy of the mess age which
includes the hash value particular to the series of sample windows
that will be used to encode the message. Sign and encrypt this copy
of the message, and save it for encoding in the same place in the
sample stream.
A memory efficient version of this scheme could keep on hand the
un-hashed message, and as it creates each new copy, back up in the
sample stream to the first window in the series and actually encode
each message, disposing of it afterwards.
The important result is evident on decoding. The decoding party can
calculate the same hash used to encode the message for themselves,
but on the encoded samples. If the value calculated by the decoding
party does not match the value contained in the signed message, the
decoder is alerted to the fact that this watermark was transplanted
from somewhere else. This is possible only with a hash function
which ignores the changes made by the stega-cipher after the hash
in the watermark was generated.
This scheme makes it impossible to transplant watermarks, even with
the keys to the stega-cipher.
APPENDIX
Pseudo-code
TABLE-US-00001 const int WINDOW_RESET = 256; const int WINDOW_SIZE
= 128; const int MARKER_BITS = 1024; const int CHUNK_BITS = 2048 *
8; int window_offset; int msg_bit_offset; int frequency_offset;
Boolean useCell; /* 8 bits per bye, 1 byte per char */ unsigned
char frequency_mask[WINDOW_SIZE/8]; unsigned char
window_mask[WINDOW_RESET/8]; unsigned char
msg_start_marker[MARKER_BITS/8]; unsigned char
msg_end_marker[MARKER_BITS/8]; Int16
amplitude_sample_buffer[WINDOW_SIZE]; float
power_frequency_buffer[WINDOW_SIZE]; unsigned char
message_buffer[CHUNK_BITS/8]; void doFFT(Int16 *amp_sample_buffer,
float *power_freq_buffer,int size); void doInverseFFT(Int16
*amp_sample_buffer, float *power_freq_buffer,int size); void
initialize( ); Bit getBit(unsigned char *buffer,int bitOffset);
Boolean map(Bit window_bit, Bit band_bit, int window, int
frequency); Boolean getSamples(Int16 *amplitude_sample_buffer,int
samples); void encode( ) void initialize( ) { /* message to be
encoded is generated */ /* message is prefixed with 1024 bit
msg_start_marker */ /* message is suffixed with 1024 bit
msg_end_marker */ /* remaining space at end of message buffer
padded with random bits */ window_offset = 0; msg_bit_offset = 0;
frequency_offset = 0; frequency_mask loaded window_mask loaded
zeroAmpSampleBuffer( ); } Boolean getSamples(Int16 *buffer,int
samples) { /* get samples number of samples and shift them
contiguously into the sample buffer from right to left*/ if(samples
< samples available) return false; else return true; } void
doFFT(Int16 *sample_buffer, float *spectrum_buffer, int size) {
calculate FFT on sample_buffer, for size samples store result in
spectrum buffer } void doInverseFFT(Int16 *sample_buffer,float
*spectrum_buffer,int size) { calculate inverse FFT on
spectrum_buffer store result in sample_buffer } Bit getBit(unsigned
char *buffer,in bitOffset) { returns value of specified bit in
specified buffer either 0 or 1, could use Boolean (true/false)
values for bit set of bit off } Boolean map(Bit window_bit,Bit
band_bit,int window, int frequency.sub.-- { /* this is the function
that makes the information difficult to find */ /* the inputs
window_bit and band_bit depend only on the mask values used for
encoding the information, they are 1) random, 2) secret */ /*
window and frequency values are used add time and frequency band
dependent complexity to this function */ /* this function is
equivalent to a Boolean truth table with window * frequency * 4
possible input combinations and 2 possible output */ /* for any
input combination, the output is either true or false */ /* window
ranges from 0 to WINDOW_RESET -1 */ /* frequency ranges from 0 to
WINDOW_SIZE - 1 */ return calculated truth value } void
encodeBit(float *spectrum_buffer,int freq_offset,Bit theBit) { /*
modifies the value of the cell in spectrum_buffer, indexed by
freq_offset in a manner that distinguishes each of the 2 possible
values of theBit, 1 or 0 */ /* suggested method of setting the
Least Significant bit of the cell == theBit */ /* alternative
method of rounding the value of the cell upward or downward to
certain fractional values proposed i.e. <= .5 fractional
remainder signifies 0, > .5 fraction remainder signifies 1 */ }
void encode( ) { initialize( ); do {
if(getSamples(amplitude_sample_buffer) == false) return
doFFT(amplitude_sample_buffer,power_frequency_buffer, WINDOW_SIZE);
for (frequency_offset = 0; frequency_offset < WINDOW_SIZE;
frequency_offset++){ useCell =
map(getBit(window_mask,window_offset),
getBit(frequency_mask,frequency_offset), window_offset,
frequency_offset); if(useCell == true){
encodeBit(power_frequency_buffer,frequency_offset,
getBit(message_buffer,msg_bit_offset)); message_bit_offset ++;
if(msg_bit_offset == MESSAGEBITS){ initialize( ); break; /* exit
frequency loop */ } } }
doInverseFFT(amplitude_sample_buffer,power_frequency_buffer,
WINDOW_SIZE); outputSamples(amplitude_sample_buffer);
window_offset++; if(window_offset == WINDOW_RESET){ window_offset =
0; } } while(true); }
The encode( ) procedure processes an input sample stream using the
specified frequency and window masks as well as a pre-formatted
message to encode.
encode( ) processes the sample stream in windows of WINDOW_SIZE
samples, contiguously distributed in the sample stream, so it
advances WINDOW_SIZE samples at a time.
For each sample window, encode( ) first compute the FFT of the
window, yielding its Power Spectrum Estimation. For each of these
window PSEs, encode( ) then uses the map( ) function to determine
where in each PSE to encode the bits of the message, which it reads
from the message buffer, on ebit at a time. Each time map( )
returns true, encode( ) consumes another sample from the
message.
After each window is encoded, encode( ) computes the inverse FFT on
the PSE to generate a modified sample window, which is then output
as the modified signal. It is important the sample windows NOT
overlap in the sample stream, since this would potentially damage
the preceeding encoding windows in the stream.
Once the message is entirely encoded, including its special end of
message marker bit stream, encode( ) resets it internal variables
to begin encoding the message once more in the next window, encode(
) proceeds in this manner until the input sample stream is
exhausted.
TABLE-US-00002 enum { Synchronizing, Locked }; /* decode states */
unsigned char message_end_buffer[MARKER_BITS]; Bit decodeBit(float
*spectrum_buffer,int freq_offset) { /* reads the value of the cell
in spectrum_buffer, indexed by freq_offset in a manner that
distinguishes each of the 2 possible values of an encoded bit, 1 or
0 * / /* suggested method of testing the Least Significant bit of
the cell */ /* alternative method of checking the value of the cell
versus certain fractional remainders proposed. i.e. <= .5
fractional remainder signifies 0, > .5 fraction remainder
signifies 1 * / return either 1 or 0 as appropriate } Boolean
decode( ) { /* Initialization */ state = Synchronizing
window_offset = 0; set frequency mask set window mask clear sample
buffer int nextSamples = 1; mt msg_start_offset = 0; clear
message_end_buffer Bit aBit; Boolean bitsEqual; do { if(state ==
Synchronizing){ nextSamples = 1; window_offset = 0; } else
nextSamples = WINDOW_SIZE; if(getSamples(amplitude_sample_buffer)
== false) return false;
doFFT(amplitude_sample_buffer,power_frequency_buffer,
WINDOW_SIZE);/* 2 */ for (frequency_offset = 0; frequency_offset
< WINDOW_SIZE; frequency_offset++){ useCell =
map(getBit(window_mask,window_offset),
getBit(frequency_mask,frequency_offset), window_offset,
frequency_offset); if(useCell == true){ aBit =
decodeBit(power_frequency_buffer, frequency_offset);
setBit(message_buffer,message_bit_offset,aBit); message_bit_offset
++; } else continue; if(state == Synchronizing){ bitsEqual =
compareBits(message_start_marker,message_buffer,
message_bit_offset); if(!bitsEqual){ message_bit_offset = 0;
misaligned = true; break; /* exit frequency loop */ } else if
(message_bit_offset == MARKER_BITS) state == Locked; } else { /*
locked onto encoded stream */ shift aBit into right side of
message_end_buffer bitsEqual = compareBits(message_end_buffer,
msg_end_marker,MARKER_BITS); if(bitsEqual) return true; } } } while
(true); }
The decode( ) procedure scans an input sample stream using
specified window and frequency masks, until it either decodes a
valid message block, storing it in a message buffer, or exhausts
the sample stream.
The decode( ) procedure starts in state Synchronizing, in which it
does not know where in the sample stream the encoding windows are
aligned. The procedure advances the sample window through the
sample stream one sample at a time, performing the FFT calculation
on each window, and attempting to decode valid message bits from
the window. As it extracts each bit using the map( ) function, the
decode( ) procedure compares these bits against the start of
message marker. As soon as a mismatch is detected, the decode( )
procedure knows it is not yet properly aligned to an encoding
window, and immediately ceases decoding bits from the current
window and moves to the next window, offset by 1 sample. The
decode( ) procedure continues in this manner until it matches
successfully the complete bitstream of a start of message marker.
At this point the decode( ) procedure assumes it is aligned to an
encoded message and can then decode bits to the message buffer
quickly, advancing the sample window fully at each iterations. It
is now in Locked mode. For each bit it stores in the message buffer
when in Locked mode, the decode( ) procedure also shifts the same
bit value into the least significant bit of the message_end_buffer.
After each bit is decoded in Locked mode, the decode( ) procedure
checks compares the message_end_buffer with the msg_end_marker in a
bit by bit manner. When a complete match is found, decode( ) is
finished and returns true. If the sample stream is exhausted before
this occurs, decode( ) returns false. If decode( ) returns true, a
valid message is stored in the message buffer, including the start
and end of message markers.
* * * * *
References